Acids, Bases and Salts Class 10 Science

20 Questions | 60 Marks

A student observes that a solution turns red litmus blue and another solution turns blue litmus red. Which solution is acidic and which is basic? Explain your reasoning using the behaviour of litmus indicators. 2marks

When 5 mL of a solution of sodium hydroxide is neutralised by 8 mL of a solution of hydrochloric acid, what is the volume of the same hydrochloric acid solution required to neutralise 10 mL of the same sodium hydroxide solution? Justify your answer. 3marks

Identify the type of reaction and the products formed in each of the following cases:(a) Zinc granules react with dilute hydrochloric acid.(b) Copper oxide reacts with dilute hydrochloric acid.(c) Sodium hydroxide reacts with hydrochloric acid.(d) Sodium carbonate reacts with dilute hydrochloric acid. 4marks

Case Study: A farmer has a soil sample from his field. He wants to grow a specific type of plant that requires a pH range of 6.5 to 7.5 for healthy growth. He tests the soil and finds its pH is 4.5.(a) Is the soil acidic or basic? [1 mark](b) What substance can the farmer add to the soil to increase its pH to the desired range? Name two such substances. [2 marks](c) Explain why maintaining the correct soil pH is crucial for plant growth, referencing the effect on nutrient absorption. 2marks

Explain why pure water does not conduct electricity, but rainwater does. 2marks

What is meant by 'water of crystallisation'? Give an example of a salt that contains water of crystallisation and write its formula. 2marks

Explain the term 'neutralisation reaction' and provide an example with a balanced chemical equation. 3marks

What is the difference between a strong acid and a weak acid? Give one example of each. 2marks

A child is suffering from indigestion after eating a spicy meal. Their stomach is producing excess acid.(a) What is the approximate pH of the stomach fluid in this condition? [1 mark](b) What type of medicine should be recommended to relieve the pain? Give the chemical name of one such medicine. 2marks

Why is it recommended to add acid to water slowly with stirring while diluting, rather than adding water to concentrated acid? 2marks

Explain the term 'alkali'. Give an example of an alkali. 2marks

How does the presence of acid rain affect aquatic life? 2marks

Write the balanced chemical equation for the preparation of Sodium Hydroxide from Sodium Chloride by the Chlor-alkali process. Name the gases evolved at the anode and cathode. 3marks

What happens when baking soda is heated during cooking? Give the balanced chemical equation for the reaction. 2marks

Name the products formed when Plaster of Paris reacts with water. Write the balanced chemical equation for this reaction. 2marks

A solution of potassium chloride is neutral. Identify the acid and base from which it is formed. 1mark

What are olfactory indicators? Give an example of a substance that can be used as an olfactory indicator. 2marks

Assertion (A): All substances containing hydrogen are not acidic. Reason (R): Acids produce hydrogen ions (

H^{+}

) in aqueous solution, which are responsible for their acidic properties.(a) Both A and R are true and R is the correct explanation of A.(b) Both A and R are true but R is not the correct explanation of A.(c) A is true but R is false.(d) A is false but R is true. 1mark

Case Study: The pH of soil is crucial for the healthy growth of plants. A gardener notices that the plants in his garden are not growing well. He tests the soil and finds that the pH is 8.5.(a) Is the soil acidic, basic, or neutral? [1 mark](b) What substance can the gardener add to the soil to make it more suitable for plant growth? Name the substance and write its chemical formula. [2 marks](c) Explain the role of pH in tooth decay. 2marks

Assertion (A): Bleaching powder is used to disinfect drinking water. Reason (R): Bleaching powder acts as an oxidising agent and kills the germs.(a) Both A and R are true and R is the correct explanation of A.(b) Both A and R are true but R is not the correct explanation of A.(c) A is true but R is false.(d) A is false but R is true. 1mark

Answers

The solution that turns red litmus blue is basic. Bases turn red litmus blue. The solution that turns blue litmus red is acidic. Acids turn blue litmus red.

The reaction between sodium hydroxide and hydrochloric acid is a neutralisation reaction: NaOH + HCl → NaCl + H2O. This reaction occurs in a 1:1 molar ratio. If 5 mL of NaOH requires 8 mL of HCl, then 10 mL of NaOH (double the volume) will require double the volume of HCl, which is 16 mL.

(a) Acid + Metal → Salt + Hydrogen gas. Zinc granules + dilute HCl → Zinc chloride + Hydrogen gas.

Z n (s) + 2 H C l (a q) \to Z n C l_{2} (a q) + H_{2} (g)

. This is a displacement reaction.(b) Metal oxide + Acid → Salt + Water. Copper oxide + dilute HCl → Copper(II) chloride + Water.

C u O (s) + 2 H C l (a q) \to C u C l_{2} (a q) + H_{2} O (l)

. This is a neutralisation reaction.(c) Acid + Base → Salt + Water. Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water.

N a O H (a q) + H C l (a q) \to N a C l (a q) + H_{2} O (l)

. This is a neutralisation reaction.(d) Metal carbonate + Acid → Salt + Carbon dioxide + Water. Sodium carbonate + dilute HCl → Sodium chloride + Water + Carbon dioxide.

N a_{2} C O_{3} (s) + 2 H C l (a q) \to 2 N a C l (a q) + H_{2} O (l) + C O_{2} (g)

. This is a displacement reaction.

(a) The soil is acidic. A pH of 4.5 is less than 7.(b) The farmer can add basic substances to increase the pH. Two such substances are quick lime (calcium oxide, CaO) or slaked lime (calcium hydroxide, Ca(OH)₂).(c) Plants require specific pH ranges for nutrient absorption. In acidic soil (low pH), essential nutrients like phosphorus may become unavailable, and toxic elements like aluminium may become more soluble, hindering root growth and overall plant health.

Pure water is a covalent compound and does not have free ions to conduct electricity. Rainwater contains dissolved minerals and gases (like dissolved CO₂ forming carbonic acid), which dissociate into ions, allowing it to conduct electricity.

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. For example, Copper(II) sulphate crystals have five water molecules. Its formula is

C u S O_{4} .5 H_{2} O

A neutralisation reaction is a reaction in which an acid and a base react with each other to form salt and water. Example: Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water.

H C l (a q) + N a O H (a q) \to N a C l (a q) + H_{2} O (l)

A strong acid completely ionizes in aqueous solution, producing a high concentration of

H^{+}

ions. Example: Hydrochloric acid (HCl). A weak acid partially ionizes in aqueous solution, producing a lower concentration of

H^{+}

ions. Example: Acetic acid (

C H_{3} C O O H

(a) The pH of the stomach fluid in this condition would be less than 7, likely around 1-2, due to excess hydrochloric acid.(b) An antacid medicine should be recommended. Examples include Magnesium hydroxide (chemical name), commonly known as milk of magnesia.

Adding concentrated acid to water generates a large amount of heat (exothermic process). If water is added to the acid, the heat generated can cause the mixture to splash out, leading to burns and potential breakage of the container. Adding acid to water allows the larger volume of water to absorb the heat gradually.

An alkali is a base that dissolves in water. They are typically bitter, soapy to touch, and corrosive. Example: Sodium hydroxide (NaOH).

Acid rain lowers the pH of river water. Aquatic organisms are sensitive to changes in pH and can die if the pH falls too low, making survival difficult.

During the electrolysis of brine (aqueous solution of sodium chloride), Sodium Hydroxide is formed. The balanced chemical equation is:

2 N a C l (a q) + 2 H_{2} O (l) \overset{e l e c t r o l y s i s}{\to} 2 N a O H (a q) + C l_{2} (g) + H_{2} (g)

. Gas evolved at the anode is Chlorine (

C l_{2}

). Gas evolved at the cathode is Hydrogen (

H_{2}

When baking soda (sodium hydrogencarbonate) is heated, it decomposes to form sodium carbonate, water, and carbon dioxide gas. The equation is:

2 N a H C O_{3} (s) \overset{h e a t}{\to} N a_{2} C O_{3} (s) + H_{2} O (l) + C O_{2} (g)

When Plaster of Paris reacts with water, it forms gypsum. The balanced chemical equation is:

C a S O_{4} . \frac{1}{2} H_{2} O (s) + 1 \frac{1}{2} H_{2} O (l) \to C a S O_{4} .2 H_{2} O (s)

Potassium chloride (KCl) is a neutral salt. It is formed from the reaction of a strong acid, hydrochloric acid (HCl), and a strong base, potassium hydroxide (KOH).

Olfactory indicators are substances whose smell changes in acidic or basic media. Example: Vanilla essence or clove oil. Their smell becomes less pronounced in basic medium, and remains unchanged in acidic medium (for vanilla).

(a) Both A and R are true and R is the correct explanation of A. Substances like glucose and alcohol contain hydrogen but do not dissociate to produce

H^{+}

ions in aqueous solution, hence they are not acidic.

(a) The soil is basic. A pH of 8.5 is greater than 7.(b) The gardener can add a weak acid to neutralise the excess base and bring the pH down. An example is Tartaric acid (

C_{4} H_{6} O_{6}

), which is present in tamarind.(c) Tooth decay starts when the pH of the mouth falls below 5.5. The tooth enamel, made of calcium hydroxyapatite, gets corroded by acids produced by bacteria after consuming food, especially sugary items. Using basic toothpaste neutralises the excess acid.

(a) Both A and R are true and R is the correct explanation of A. Bleaching powder contains chlorine and has oxidising properties, which help in killing germs.

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Acids, Bases and Salts — Important Questions

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